This invention relates to the control of an internal combustion engine having variable cam phasing, and more particularly to a method of estimating the concentration of residual exhaust gas in an engine cylinder for engine control purposes.
Accurate control of engine fuel injection and spark timing requires knowledge of the concentration of exhaust gas in the engine cylinders during the combustion stroke. Such exhaust gases may be present either due to external recirculation in which an EGR valve establishes a variably restricted passage between the engine intake and exhaust manifolds, and/or internal (i.e., residual) recirculation in which the timing of the intake and exhaust valve openings permits a portion of the exhaust gases to remain in the engine cylinders. The concentration of exhaust gas due to external recirculation can be estimated fairly reliably based on the EGR valve position and the gas pressures in the intake and exhaust manifolds. On the other hand, the concentration of residual exhaust gas due to internal recirculation is difficult to reliably estimate, particularly when the engine is equipped with a mechanism for adjusting the phase of the intake and/or exhaust valve timing since such adjustment alters the breathing characteristics of the engine. For this reason, engine control functions that are sensitive to the cylinder exhaust gas concentration topically include various gains and/or offsets that are calibrated to compensate for the effects of residual exhaust gas. However, this approach is not particularly desirable since separate calibration values are required for each such control function, and a fairly intensive effort is required to tune the several calibration values for a given engine. Theoretically, the calibration effort could be greatly simplified by directly estimating the concentration of residual exhaust gas (using test data obtained from engine dynamometer testing or software simulation, for example), but such data is difficult to measure and subject to open-loop simulation error. Accordingly, what is needed is a reliable and easily calibrated technique for directly estimating the concentration of residual exhaust gas based on engine dynamometer data and engine simulation data.
The present invention is directed to an improved method of estimating the concentration of residual exhaust gas in an internal combustion engine having variable cam phase control based on engine dynamometer and software simulation data. Essentially, the invention recognizes that both the volumetric efficiency and the residual exhaust gas concentration of an engine vary monotonically in response to changes in cam phase, and that the simulated residual concentration data will be reliable if the simulated volumetric efficiency data matches volumetric efficiency data determined by engine dynamometer testing. Thus, volumetric efficiency test data is compared to simulated volumetric efficiency data, and the simulation software is tuned until the simulated data matches the test data. At such point, the simulated residual concentration data is deemed to be reliable, and is used to calibrate a model relating residual concentration to cam phase angle, and such model is then used by an engine controller to estimate residual exhaust gas concentration during operation of the engine.